Printed circuit boards can fail by breaks in the copper conductive pattern caused by the stresses induced by the difference between the thermal expansion of the copper conductive pattern and the insulating substratum to which it is attached. Thermal stresses occur when components are joined to the conductive pattern by soldering at temperatures from 200.degree. to 300.degree. C. The printed circuit industry has recognized this problem by imposing thermal stress test and tensile requirements on printed circuit boards.
MIL-P-55110 D specifications require the printed circuit boards to be thermally stressed by application of molten solder at 288.degree. C. for ten seconds. After soldering, holes in the test board are microsectioned and examined with a 100x microscope. The resistance to thermal stress is evaluated by assigning a crack rating on a scale of 0 to 160 with ratings above 15 being considered a failure of the test. Since there may be more than one soldering cycle in the assembly of a printed circuit, some electronic equipment manufacturers have required suppliers of printed wiring boards to test the boards by four repetitive thermal stress cycles at 288.degree. C.
IPC TECHNICAL REPORT, IPC-TR-579, ROUND ROBIN RELIABILITY EVALUATION OF SMALL DIAMETER PLATED THROUGH HOLES IN PRINTED WIRING BOARDS, Lincolnwood, Ill.: Institute for Interconnecting and Packaging Electronic Circuits, 1988, p. 25, describes thermal cycling between 25.degree. C. and 260.degree. C. in a Fluid Sand T-Shock test. The boards are cycled to failure, which is an open circuit or a 10% increase in the resistance of a chain of plated-through holes. Five cycles without failure is considered acceptable.
Printed circuit boards comprise epoxy resin dielectrics adjacent to copper surfaces for various functions. Joining layers between inner layer conductors of multilayer printed circuit boards comprise epoxy resin containing polymers. Epoxy resin dielectrics are often used as solder resists and as plating resists for defining conductor patterns. In the manufacture of printed circuit boards by a partially additive process the resists are required to firmly adhere to copper surfaces even after several hours of plating in an electroless copper solution at elevated temperatures and high pH.
It is difficult to permanently bond copper surfaces and such resists together. The electroless plating solutions employed have a tendency to penetrate between the copper surface and the resist film due to the long duration of electroless plating. The resist films lift during plating, leaving voids and allowing plating solutions to penetrate between the copper surface and the resist layer, thus rendering the resulting printed circuit board unreliable.
To avoid these problems, methods have been disclosed in which adhesion promoting substances are employed, either between the copper surfaces and the resist layers, or as components of the resist layers to be adhered to the copper surfaces.
An elucidation of the partially additive process to which the present invention is applicable, to the extent that the subsequent description does not specifically describe such a process, is to be found in Printed Circuits Handbook, 3rd Edition, Chapter 13, Clyde F. Coombs, ed., McGraw-Hill, New York (1988), which is incorporated herein by reference.
U.S. Pat. No. 5,028,513 to Murakami et al. discloses the addition of benzotriazole and other related compounds into a plating resist for improving adherence of the resist to a copper conductor. Murakami et al. recognize the deleterious effects of benzotriazole and certain S-triazine compounds on the performance of an electroless copper bath and the adverse affects of these same compounds on the electroless copper deposits formed therefrom. There is however, no teaching in Murakami et al. for applying S-triazine compounds directly to the surface of unroughened copper. Moreover, Murakami et al. suggest the use of sublimable S-triazine compounds, while the present invention does not require such usage.
U.S. Pat. No. 5,091,283 to Tanaka et al. discloses the incorporation of S-triazine compounds into plating resists useful in a partially additive manufacturing process. Tanaka et al. recognize that such plating resists may peel from copper surfaces immersed in an electroless plating solution, however Tanaka et al. teach the use of the S-triazines in combination with dicyandiamide solely as curing agents for the epoxy resin component of a dialkyl phthalate resist, and there is no recognition that the S-triazine derivatives are useful as adhesion promoting agents.
In another process, U.S. Pat. No. 4,693,959 to Ashcraft discloses plating resists comprising adhesion promoting polymers formed by condensation of formaldehyde and S-triazine compounds, and preferably a condensation polymer of formaldehyde and melamine. There is no teaching in Ashcraft for the use of such plating resists in electroless copper solutions.
In U.S. Pat. No. 5,100,767 to Yanagawa et al. S-triazine compounds are disclosed as latent photoresist hardeners which also have the effect of strengthening adhesion of the photoresist resist to copper. There is no recognition that S-triazine compounds will affect the performance of the electroless plating bath or the physical properties of the copper deposits, nor does Yanagawa et at. teach methods for circumventing these problems.
Partially additive processes using resists comprising S-triazine compounds have proven difficult to control. When the S-triazine compounds are employed in low concentration, or not present at all, adherence of the resist to the copper during subsequent electroless plating is poor.
At effective levels for promoting cure of the resist, elution of the adhesion promoting compounds from the resists into the electroless plating solutions results in the formation of copper films which are brittle and not useful for printed circuits.
Up to the time of the present invention, it has not been possible to use resists comprising S-triazine compounds of the type disclosed in Tanaka et al. in the manufacture of thermal stress resistant copper by a partially additive process. The S-triazine elutes into the electroless plating solution and the resultant copper deposits fail under thermal stress.